The complete combustion of fuel, especially fuel from such diverse and low quality sources as low grade fuels and municipal and industrial waste, to generate usable power without concomitantly creating environmental problems has remained an elusive goal yet to be fully achieved. Low grade fuels tend to contain high amounts of sulfur and produce the sulfur oxides implicated in acid rain when they are burned in power plants. Municipal and industrial waste has traditionally been disposed of in landfills. However, not only is there an acute shortage of landfill areas, but burying this waste has been demonstrated to contaminate the surrounding soil and adjacent ground water with toxic substances. Some industrial waste, such as that containing low level radiation, for example, is particularly hazardous and difficult to dispose of without creating environmental or health problems.
Many industrial plants and communities have resorted to burning their wastes. Although recycling programs have produced some reductions in the quantity of municipal and industrial waste to be disposed of, a vast quantity of such waste still remains to be disposed of by burning or otherwise. The burning or incineration of waste presents many serious problems. Existing incineration plants inevitably discharge pollutants in excess of clean air standards, despite efforts to minimize the discharge of these pollutants by the use of scrubbers, filters and electrostatic precipitators. Moreover, the incineration process generates toxic ash, which must be disposed of.
The waste management industry has employed wet oxidation reactors for the combustion of organic wastes, including those generated by garbage, pulp and paper operations and milk processing, to produce mainly carbon dioxide and water. A typical process introduces a waste stream and air to a reactor at high temperature and pressure, which results in the oxidation of the organic matter in the waste stream in an exothermic reaction that may generate usable heat if the reaction is controlled. Low grade solid fuels may also be burned in a wet oxidation reactor. The careful control and monitoring required for wet oxidation reactors has been one of their major disadvantages. In addition, the available wet oxidation reactors are thermally inefficient and do not produce substantially complete combustion of the organic wastes or fuel. U.S. Pat. Nos. 4,053,404 to Van Kirk; 4,384,959 to Bauer et al.; 4,670,162 to Robey; and 4,721,575 to Binning et al. are illustrative of prior art wet oxidation reactors and wet oxidation reaction processes.
The prior art has proposed combustion systems for municipal waste, coal, and the like which permit the recovery of valuable resources and produce energy useful for power generation or in a heat exchanger. U.S. Pat. Nos. 3,986,955 to Plique and 4,829,911 to Nielson are exemplary of such systems. These systems, however, are not thermally efficient because the wet oxidation process provides the only combustion system and is not properly controlled to effect complete combustion. As a result, the maximum operating temperatures are not likely to be high enough to allow substantially complete combustion of the organic reactants or to generate usable energy.
The inclusion of additional combustion stages in various combustion processes to effect the substantially complete combustion of hydrocarbon fuels has been proposed in U.S. Pat. Nos. 4,240,784 to Dauvergne; 5,215,455 to Dykema and 5,271,729 to Gensler et al. However, none of these patents suggests improving the combustion conditions in a wet oxidation reactor or direct contact boiler with additional combustion stages to enhance the thermal efficiency of the combustion of varied mixtures of organic waste materials and low quality fuel.
U.S. Pat. No. 4,700,637 to McCartney discloses a two stage combustion or incineration process for the substantially complete combustion of low level radiation waste which includes the addition of a supplemental conventional fuel and air in excess of that required for stoichiometric combustion. Although it is disclosed that liquid waste can be processed by this system, it is essentially a dry incineration process, and there is no suggestion that it could be used in connection with a wet oxidation process or direct contact boiler to produce usable power at enhanced thermal efficiency while producing clean, nonpolluting efflux.
The prior art, therefore, has failed to provide a combustion system employing a wet oxidation reactor or a direct contact boiler and a stoichiometric second stage burner which promotes the substantially complete combustion of a range of organic materials from municipal waste to low quality fuels at a thermal efficiency which produces usable energy for power generation without producing undesirable environmental pollutants. A need exists for such a combustion system.